The biological response to a noxious stimulus is not pain. The biological sensory/discriminative awareness of noxious stimuli is called nociception. In a clinical environment it is important to differentiate this principal from the patients expression of what they feel, which is pain and to therefore determine the pathological cause of their pain and to deliver the correct treatment. In addition there are events in the brain associated with changes such as respiration and blood pressure, which follow a noxious stimulus, not related to pain.
Enhancement of pain transmission: Neurophysiologic studies of pain modulating neurons in the brain stem have revealed that there are two types of nerve cells involved in pain control. (i) ‘Off cell’ excited by morphine, its firing pattern inhibits pain transmission-analgesia. (ii) ‘On cell’ enhances pain transmission, these cells can be shut off by analgesic actions of morphine. Thus, bi-directional control of pain transmission. Hyperalgesia (increased pain levels) could result from either the loss of inhibition or the release of excitatory modulating neurons (‘on cells’) from inhibitory control (which results in these increased pain levels).